Radio frequency (RF) transmitter-receivers and transceivers have been capable of both transmitting and receiving RF signals. Typically, the transmit bands and the receive bands have been offset from each other to minimize interference at the receiver from the outgoing transmitter signals. Other interference reducing techniques, such as the use of special filters such as surface acoustic wave (SAW) filters have also been used to remove unwanted frequency components from the transmitted signals and reduce interference at the receiver.
Many transceivers today use digital predistortion techniques to generate more usable power from amplifiers, avoiding the need for larger amplifiers consuming more power. However, the use of these digital predistortion techniques caused additional transmitter noise to leak into the received signals at the receiver. The additional noise leakage has been reduced by increasing the size of the duplexer to achieve better isolation between transmit and receive bands. However, as wireless devices such as phones, tablets, and other RF devices become smaller and less expensive, these larger and more expensive duplexers have become impractical.
To reduce this additional transmitter noise, an estimate of the noise from the transmitted signals that is expected to leak can be generated. This estimate can then be used to reduce the additional transmitter noise. This process is sometimes called transmit noise cancellation. However, generating the estimate of leakage noise typically requires additional and redundant receiver circuitry. Moreover, to extend conventional transmit noise cancellation to multi transmitter-receiver systems, additional receiver circuitry will be needed for each transmitter-receiver pair. Again, as wireless devices become smaller and less expensive, the size cost of additional receiver circuitry can become impractical.
Therefore, there is a need for a transmit noise cancellation system that can be extended to multi transmitter-receiver systems without the extra cost of having additional receiver circuitry for each transmitter-receiver pair.